A tapered roller bearing and an angular contact ball bearing are used in such a state that an axial preload is applied thereto. For example, in an automotive gear-type drive transfer unit such as a transmission, a tapered roller bearing is adopted in its main part (for example, a final drive portion in the transmission), and as is shown in FIG. 7(a), a rotational shaft 115 is press fitted in an inner ring 133 of a taper roller bearing 111, and an outer ring 132 is press fitted in a bearing housing 125 of a transmission case. Thereafter, a preload is made to be imparted towards one side (an arrow a) in an axial direction. When the preload is imparted, the outer ring 132 receives a component of force on an inclined rolling surface of a tapered roller 134 and is displaced in the axial direction and a radial direction, and its right end face 132c and outer circumferential surface 132b are pressed against an inner end face 125c and an inner circumferential surface 125a of the bearing housing 125, respectively, whereby the preload is supported.
On the other hand, in recent years, as part of the weight reduction, it is adopted that a transmission case (a bearing housing) is made of a light metal such as an Al alloy. In structural materials, Al has a highest linear thermal expansion coefficient (about 23.5×10−6/° C. at room temperatures: hereinafter, the unit of linear thermal expansion coefficient will be expressed as ppm/° C. in an abbreviated shortened), and there is a considerable difference between the linear thermal expansion coefficient of Al and a linear thermal expansion coefficient (about 12 ppm/° C. at room temperatures) of a steel (an Fe-based material) which makes up a rotational shaft or a tapered roller bearing.
In a case where the rotational shaft and the bearing housing are made of the same material, since a change in dimension due to temperature thereof becomes the same, there is no large change in preload which is applied to the tapered roller bearing. However, in the event that the bearing housing is made of a light metal, the bearing housing changes in dimension larger than the rotational shaft as the temperature of the transmission increases, and this may cause a fear that the preload is lost.
Specifically, as is shown in FIG. 7(b), when the temperature of the transmission increases, although the bearing housing 125 and the rotational shaft 115 expand, an inner circumferential raceway surface 132a of the outer ring 132 is separated from the rolling surface of the tapered roller 134 in a direction indicated by an arrow b due to a difference in dimension change caused by the expansion. Namely, an axial space and a radial space of the tapered roller bearing 111 change largely due to the increased temperature, and the preload becomes insufficient. Such an insufficient preload calls for looseness in the gears and becomes a cause for generation of noise.
As a means for solving the problem above, Patent Document 1 below discloses a rolling bearing apparatus in which a preload is imparted to an outer ring by hydraulic pressure or a spring. Specifically, a bottomed cylindrical cylinder is formed in a bearing housing, and an outer ring is fitted in this cylinder so as to slide in an axial direction, while a disk-shaped preload member is provided so as to be brought into abutment with an axial outer end portion of the outer ring, so that oil is supplied into an oil pressure chamber which is surrounded by a cylinder inner surface and the preload member by a hydraulic pump. Further, a compression coil spring is provided within the oil pressure chamber so as to bias the preload member inwards in the axial direction.
In this configuration, while the preload is imparted to the preload member by the oil pressure and the compression coil spring, when the bearing housing changes in dimensions larger than the bearing housing and a rotational shaft due to an increase in temperature, the outer ring can be moved inwards in the axial direction via the preload member by the functions of the compression coil spring and the oil pressure, so as to suppress a change in axial space and radial space of the tapered roller bearing to thereby solve the problem of the insufficient preload.    Patent Document 1: JP-A-2006-153090